This invention relates to rotating anode x-ray tubes. In known devices the rotor assembly is commonly supported by a pair of axially spaced apart self lubricating ball bearings having inner races fixed to the rotor shaft and outer races fixed to a stationary tubular stem. Such a structure is shown in U.S. Pat. No. 4,272,696, issued to the present inventor and others on June 9, 1981 and owned in common with the present invention. A major cause of bearing failure in such x-ray tubes (particularly for the front bearing, which is nearest the rotating anode) is the great amount of heat conducted through the bearings; the bearings cannot tolerate a temperature greater than about 500 degrees Celsius during tube operation. Tube operating times are thus limited by the need to prevent the bearings from failing as a result of exceeding their operating temperatures.